The present invention relates to devices for use in actuating an explosive charge downhole in a wellbore.
Explosive charges are utilized in wellbores to perform various functions, for example, to perforate a well casing to complete or test a formation, or to set a packer or other device downhole. Due to the time and expense involved in these operations and the explosive power of these devices, it is essential that their operation be reliable. The typical wellbore environment poses severe difficulties for the operation of explosive devices downhole, which thus tends to reduce their reliability. For example, extremes of temperature are common which tend to degrade the operation of explosives, and the presence of heavy drilling muds and debris can interfere with a firing apparatus. Impact responsive firing heads can become fouled by debris and particles settling out from the drilling mud.
In some applications, it is not feasible to utilize an impact responsive firing head. In drill stem testing, a zone to be tested is perforated and various downhole parameters such as temperature and pressure are monitored by instruments mounted between the tubing and the firing head. These are non-fullbore opening devices which typically do not permit a detonating bar to pass through to the firing head. In these applications, therefore, pressure responsive firing devices are desired for use.
A complication introduced in the use of pressure responsive firing, devices is that they require the manipulation of pressure in the annulus or the tubing to actuate the firing device. There are, however, numerous applications which call for the maintenance of a relatively low pressure at the time of explosive actuation, such as where it is desired to perforate the casing underbalanced. This requirement may not be compatible, therefore, with the use of pressure responsive firing devices operated by increasing pressure above hydrostatic.